102 Chapter 4
| CHECKPOINT
6a. Describe the first and second laws of
thermodynamics. Use these laws to explain why the
chemical bonds in glucose represent a source of
potential energy and describe the process by which
cells can obtain this energy.
6b. Define the terms exergonic reaction and endergonic
reaction. Use these terms to describe the function of
ATP in cells.
7a. Using the symbols X- H 2 and Y, draw a coupled
oxidation-reduction reaction. Designate the molecule
that is reduced and the one that is oxidized and state
which one is the reducing agent and which is the
oxidizing agent.
7b. Describe the functions of NAD, FAD, and oxygen (in
terms of oxidation-reduction reactions) and explain
the meaning of the symbols NAD, NADH 1 H 1 , FAD,
and FADH 2.The sudden onset of Sheryl’s great fatigue and disori-
entation is cause for concern and warranted immediate
medical attention. Examination of table 4.1 with refer-
ence to the disorders indicated by elevated levels of
CK, LDH, AST, and ALT reveal that they share one pos-
sible cause in common—myocardial infarction (heart
attack). This possibility is reinforced by the laboratory
tests demonstrating that she had elevated levels of the
CK-MB isoenzyme, which is released by damaged heart
cells, rather than the CK-BB or CK-MM isoenzymes. A
possible myocardial infarction could explain Sheryl’s
sudden onset of symptom while performing the intense
exercise of skiing.
See additional chapter 4 Clinical Investigation on Enzyme
Tests to Diagnose Diseases in the Connect site for this text.Clinical Investigation SUMMARY
Summary
4.2 Control of Enzyme Activity 92
A. The activity of an enzyme is affected by a variety of factors.
1. The rate of enzyme-catalyzed reactions increases with
increasing temperature, up to a maximum rate.
a. This is because increasing the temperature increases
the energy in the total population of reactant
molecules, thus increasing the proportion of reactants
that have the activation energy.
b. At a few degrees above body temperature, however,
most enzymes start to denature, which decreases the
rate of the reactions that they catalyze.
2. Each enzyme has optimal activity at a characteristic
pH—called the pH optimum for that enzyme.
a. Deviations from the pH optimum will decrease the
reaction rate because the pH affects the shape of the
enzyme and charges within the active site.
b. The pH optima of different enzymes can vary
widely—pepsin has a pH optimum of 2, for example,
while trypsin is most active at a pH of 9.
3. Many enzymes require metal ions in order to be active.
These ions are therefore said to be cofactors for the
enzymes.
4. Many enzymes require smaller organic molecules for
activity. These smaller organic molecules are called
coenzymes.
a. Coenzymes are derived from water-soluble vitamins.
b. Coenzymes transport hydrogen atoms and small
substrate molecules from one enzyme to another.4.1 Enzymes as Catalysts 89
A. Enzymes are biological catalysts.
- Catalysts increase the rate of chemical reactions.
a. A catalyst is not altered by the reaction.
b. A catalyst does not change the final result of a
reaction. - Catalysts lower the activation energy of chemical
reactions.
a. The activation energy is the amount of energy needed
by the reactant molecules to participate in a reaction.
b. In the absence of a catalyst, only a small proportion
of the reactants possess the activation energy to
participate.
c. By lowering the activation energy, enzymes allow a
larger proportion of the reactants to participate in the
reaction, thus increasing the reaction rate.
B. Most enzymes are proteins. - Protein enzymes have specific three-dimensional shapes
that are determined by the amino acid sequence and,
ultimately, by the genes. - The reactants in an enzyme-catalyzed reaction—called
the substrates of the enzyme—fit into a specific pocket
in the enzyme called the active site. - By forming an enzyme-substrate complex, substrate
molecules are brought into proper orientation and
existing bonds are weakened. This allows new bonds to
be formed more easily.